1. Field of the Invention
The present invention relates to a low-noise amplifier which incorporates a through mode of operation and which is constituted by only transistors with breakdown voltages of less than the power supply voltage.
2. Background of the Art
Wireless communications equipment (for example, a cellular phone) is provided, at the first stage of its receiver, with a low-noise amplifier. In the case that the wireless communications equipment is located far away from a base station (in the case of weak signals), it is required that the low-noise amplifier should exhibit a low noise characteristic and a high gain characteristic. On the other hand, in the case that the wireless communications equipment is located in the proximity of a base station (in the case of large signals), it is required that the low-noise amplifier should exhibit a low strain characteristic and a low gain characteristic. Since, in particular in mobile communications, the strength of signal varies considerably depending on the distance between a base station and a mobile station, this requires the receiver to have a wide dynamic range. To this end, it is necessary that the low-noise amplifier in the receiving front end section should incorporate a gain control function.
As prior art Example 1 (see, for example, Japanese Laid Open Patent Application Publication No. 2007-158975), there has been known a high-frequency variable gain amplifier. This amplifier includes a first amplifier (an amplification mode of operation) for high-gain amplification of high-frequency signals inputted from outside and a second amplifier (a through mode of operation) for low-gain pass of high-frequency signals inputted from outside.
Referring to FIG. 7, a description will be given in regard to the high-frequency variable gain amplifier of the prior art Example 1.
In FIG. 7, the first amplifier (amplification mode) is a cascode amplifier which comprises a cascode connection of a transistor 51 and a transistor 50, an impedance element 6 and a load impedance element 5. In addition, the second amplifier (through mode) is a through pass amplifier which comprises a series connection of a high-frequency switch 62 and an impedance element 9. When the high-frequency variable gain amplifier is made to function as the first amplifier (amplification mode), a high-frequency switch 60 is turned on, whereas the high-frequency switch 62 is turned off. The received signal, inputted to an input terminal P10, enters the base of the transistor 51, passes through the transistor 50 and then through the impedance element 7 and then through the impedance element 8, and is outputted through an output terminal P12. The factor of amplification in this case is determined by the impedance elements 6, 5.
On the other hand, when the high-frequency variable gain amplifier is made to function as the second amplifier (through mode), the high-frequency switch 60 is turned off, whereas the high-frequency switch 62 is turned on. The received signal, inputted to the input terminal P10, enters the source side of the high-frequency switch 62, thereafter is outputted through the drain side of the high-frequency switch 62, and passes through the impedance element 9. At this time, the signal path between the load impedance element 5 and the output impedance element 7 is being placed in a state of disconnection by the high-frequency switch 60. Therefore, the high-frequency signal, after passage through the impedance element 9, passes through the impedance element 8 and is provided to the output terminal P12, without leaking in the direction of the impedance element 7.